will it take off?

[Piedmont3]

The force that is generated by the planes engine is used to turn the conveyor backwards at the same speed.
The force that is generated by the planes engine is trying to move the plane forward. The conveyor is not allowing the plane to move forward because it is moving backward at the same speed, because the wheels of the plane(trying to move forward) are touching the conveyor(moving backward) the plane basically hovers on the conveyor just as someone walking on a treadmill hovers in the same spot even though they are walking with forward motion.

 

[rback33]

The force that is generated by the planes engine is used to turn the conveyor backwards at the same speed.
The force that is generated by the planes engine is trying to move the plane forward. The conveyor is not allowing the plane to move forward because it is moving backward at the same speed, because the wheels of the plane(trying to move forward) are touching the conveyor(moving backward) the plane basically hovers on the conveyor just as someone walking on a treadmill hovers in the same spot even though they are walking with forward motion.

Yes... the plane cannot pull itself through the air until it leave the ground. Highbeam tripped himself with the trickery comment... it's on the MCB.. the ground that's moving at -5x if you will when the plane tries to go +5x down the "runway" (MCB). The result is obviously 0x. NOw it's true that the props or engines pull the plane through the air, but if there is an equal force counteracting then it will never fly. The scenario could have been a matching tailwind that prevents airflow over the wings sufficient enough to overcome gravity. The upward force of airflow must exceed gravity before the plane can advance through the airmass. It can move some of it, but not enough to generate lift because of th negative force of the MCB. I am now standing by my previous theory that the plane will not fly until the MCB cause enough airflow to generate lift. I don't want to be anywhere near it when it happens.

 

[SPYDERLK]

I keep waiting for the naysayers to explain just what happens to the thrust of the engine (either prop or jet). Remember it is acting on the surrounding air which is not interacting with the MCB, i.e., if the air is calm it will remain calm no matter the speed or direction of the MCB. So the prop spins up, develops thrust trying to move the plane _through the air_. How do you who maintain the plane won't move (through the air) explain away the simple physics of the above? You somehow have to get rid of some pretty massive force trying to pull the plane _through the air_.

It is a simple equation where force = thrust. You have force, what are you going to do with the 'thrust'?

Harry K

Ok. Here is an experiment you can do that will show you how the conveyor can counter the thrust of the plane via the wheels:
* take a wheel with bearings and put it on an axle
* put the axle on 2 level rails with enuf room so the wheel is suspended between
* attach something to the wheels outer perimeter that will give you some purchase to push against - - this is your traction
* push on this "thing" gently and parallel to the rails - with just a little force the wheel will turn and your traction handhold will slip by you hand - the axle will probably not move at all
* now push on the thing hard - you will probably have to strike it to be able to deliver the force before the thing rotates out of the way - the axle will move along the rails
* Now imagine that you could continue delivering the instant force of your blow continuously as the wheel turned. The continuous rotational acceleration imparted to the wheel would continue to deliver force to the axle, pushing the axle along the rails.

The accelerating conveyor can do this to the wheel. The force it exerts to continue accelerating the wheel is transmitted to the plane via the axle. With a high enuf rate of acceleration this force can completely counteract the planes thrust. -- So, in the realm of what ifs - If the wheel to conveyor traction were great enuf to impart the needed force to accelerate the wheel(s) that fast, and if the conveyor were physically capable of doing so, then the plane would not move. Until the wheels exploded - - but if they didnt..........?
Larry

 

[patrick_g]

joerocker, While you're up you forgot to say folks apparently don't know the difference in the way a person on a treadmill propells himself and what propels the aircraft.

I have never personally known of an aircraft that drove the wheels with power to help run it down the runway nor have I known a person to propel himself by pushing air.

There have been various attempts to drive the wheels of an A/C but it was to get them up to speed prior to touchdown when landing to reduce landing shock and wear.

Pat

 

[rback33]

joerocker, While you're up you forgot to say folks apparently don't know the difference in the way a person on a treadmill propells himself and what propels the aircraft.

I have never personally known of an aircraft that drove the wheels with power to help run it down the runway nor have I known a person to propel himself by pushing air.

There have been various attempts to drive the wheels of an A/C but it was to get them up to speed prior to touchdown when landing to reduce landing shock and wear.

Pat

Really... the power DOES drive the wheels.. in a way.... the energy is being transferred through the entire plane to the axles.. this scenario could be the same whether on wheels or skids. With skids, their is a higher coefficient of friction that must be overcome between the plane and the MCB. Forward thrust and the conveyor cancel each other out because you never get the airflow to overcome the gravity...

Pat.. about you sig.... My wife grew up a city girl... Now I knew she was converted one day 4 years ago.. I was coming home from helping an area farm plow wheat fields when I happened onto a Pick-up vs deer collision that had JUST occurred. The guys radiator was gone and he was waiting for Law enforcement to get there... deer is still in the lane of traffic. so I call my wife to tell her I'm going to be a little late for dinner, as I need to get the deer outta the road... as I am walking back to remove the carcass from the road she asks me, "is any of the meat any good?" The fact that she asked it even caught HER off guard. I was impressed... btw the answer was a resounding no.

 

[NorthwestBlue]

NorthwestBlue et al, we only need enuf tire traction to prevent the plane moving with locked brakes. Tire and bearing friction losses can be zero and the plane can still be held still by the magic conveyor if the wheels are not massless. To hold the plane still the conveyor would just push backward on the tire contact patch an amount equal to the thrust of the plane engine. This would, of course, spin up the wheels very quickly to an rpm where they would fly apart. Until this happened tho the planes thrust would be counteracted by the force=MA being applied to spin up/accelerate the wheel mass. The plane would remain still. After the centrepital explosion the plane would be pretty torn up by flying debris and whats left of it would flop down on the conveyor that would be running backward at somewhat less than mach 1. Takeoff would not be an issue at this point.
Larry

Larry,

We're taking off here. Don't need brakes, don't want brakes.

 

[NorthwestBlue]

Yeah, I believe it. Lotsa good reasons probably. A plane like that may have a high landing speed that would exacerbate the backward jolt coming from quick wheel spinup on ground contact. This could cause transient stability problems in addition to the high wear and stresses.
Larry

Larry,

there is no backward jolt on landing, at least not measurable on an airpeed indicator. There is an almost (Struts, tires, ground all absorb impact) instantaneous arresting of vertical acceleration, but forward velocity remains fairly constant (minus air drag and a very small amount of rolling resistance).


Ok, now somebody help me here. I must be missing something, it's been a long time.

The force due to friction can be determined using various forms of the basic equation: F=muN

where mu (greek letter) is the coefficient of friction. It varies for static or moving (kinetic) and for differing materials

N is the normal force (in Newtons) which is the mass multiplied by the acceleration due to gravity (9.81m/s^2)

Empty weight for U2 in my previous example = 6800kg

Pilot and fuel = 1200kg

Total A/C weight = 8000kg

mu for rubber is .5 to 1.0

mu for rolling resistance hard rubber is .01 to .015

mu for steel bearings is .001

mu for Teflon is .04

So, we all seem to agree that the wheels will roll at some point after thrust is applied. Let's use mu for rolling of .015

F=muN
F=(.015)*8000kg*9.81m/s(squared)=1177.2 N

The standard U-2R with one Pratt & Whitney J75-P-13B turbojet has a sea level thrust of 76kN (17000 lbf)

76000-1177.2=74822.8 N

So we have 74.8kN of thrust to oppose this magical contraption. Throw in that F100 engine in my example with 128.9kN of thrust for even more fun.

Air drag is inconsequential because lift will cancel it out as the airplane accelerates through the air and FLYS.

So can anyone see where I went wrong on my thinking.

Oh, if anyone cares I think RobS answered the original question correctly way back on post #5.

 

[Highbeam]

Ah, the major misunderstanding that I see is that folks believe that the wheels actually have something to do with propelling a plane forward. I think everyone agrees that if the plane is not actually moving then the plane won't fly.

The wheels are just there for the ride, the engine is not powering the wheels. The wheel rotation is independent of the plane's prop providing a force to accelerate the plane.

 

[rback33]

Larry,

there is no backward jolt on landing, at least not measurable on an airpeed indicator. There is an almost (Struts, tires, ground all absorb impact) instantaneous arresting of vertical acceleration, but forward velocity remains fairly constant (minus air drag and a very small amount of rolling resistance).


Ok, now somebody help me here. I must be missing something, it's been a long time.

The force due to friction can be determined using various forms of the basic equation: F=muN

where mu (greek letter) is the coefficient of friction. It varies for static or moving (kinetic) and for differing materials

N is the normal force (in Newtons) which is the mass multiplied by the acceleration due to gravity (9.81m/s^2)

Empty weight for U2 in my previous example = 6800kg

Pilot and fuel = 1200kg

Total A/C weight = 8000kg

mu for rubber is .5 to 1.0

mu for rolling resistance hard rubber is .01 to .015

mu for steel bearings is .001

mu for Teflon is .04

So, we all seem to agree that the wheels will roll at some point after thrust is applied. Let's use mu for rolling of .015

F=muN
F=(.015)*8000kg*9.81m/s(squared)=1177.2 N

The standard U-2R with one Pratt & Whitney J75-P-13B turbojet has a sea level thrust of 76kN (17000 lbf)

76000-1177.2=74822.8 N

So we have 74.8kN of thrust to oppose this magical contraption. Throw in that F100 engine in my example with 128.9kN of thrust for even more fun.

Air drag is inconsequential because lift will cancel it out as the airplane accelerates through the air and FLYS.

So can anyone see where I went wrong on my thinking.

Oh, if anyone cares I think RobS answered the original question correctly way back on post #5.

It's been ten yrs since my last physics class, but you have not accounted for the negative movement of the MCB? The MCB matches the plane in the negative 100% of the time. While on the ground (MCB) airspeed and ground speed are the same thing because you are moving through the air but you HAPPEN to still be on the ground till you gain ENOUGH airspeed to get the bird in the air. I think we can agree this is true. On this MCB you never have ground speed nor do you have airspeed. You're dead locked in neutral.

 

[NorthwestBlue]

It's been ten yrs since my last physics class, but you have not accounted for the negative movement of the MCB? The MCB matches the plane in the negative 100% of the time. While on the ground (MCB) airspeed and ground speed are the same thing because you are moving through the air but you HAPPEN to still be on the ground till you gain ENOUGH airspeed to get the bird in the air. I think we can agree this is true. On this MCB you never have ground speed nor do you have airspeed. You're dead locked in neutral.

But, I did. As soon a the thrust is applied by the plane the MCB attempts to restrict its movement with its 1177.2 N of available friction. The thrust is greater than the retarding force (friction) and therefore the plane accerlerates relative to the ground that the MCB rests on and the air mass that surrounds both objects.